1. Field of the Invention
The present invention refers to a device with a capacitance that can be switched between at least two different values and to a non-volatile memory device using the same.
2. Description of the Related Art
For memory devices and for numerous other applications, bistable devices or circuits are used. For example, for storing one bit of information in a memory, a bistable device can be used which is switchable between (at least) two different and persistent states. When writing a logical “1” into the device, it is driven into one of the two persistant states and when writing a logical “0” or erasing the logical “1”, the device is driven into the other of the two different states. Each of the states persists until a next step of writing information into the device or erasing information in the device proceeds. A huge number of such bistable devices arranged in one or more arrays may form an EEPROM (EEPROM stands for electrically erasable programmable read-only memory) as a separate memory device or as part of an even more complex device.
An example for such a bistable device is a capacitor with a ferroelectric layer between its electrodes. The ferroelectric layer shows hysteresis polarization. The application of an electrostatic field causes a polarization of the ferroelectric material which persists after switching off the electrostatic field. The persistent remanence-polarization is deleted or erased or even inverted by an electrostatic field of opposite sign and sufficient strength.
When the ferroelectric layer is arranged between a gate and a channel of a field effect transistor, the polarization state of the ferroelectric layer influences the number of charge carriers induced in the channel. When the ferroelectric layer is positioned between the electrodes of a capacitor, the polarization state influences the capacitance of the capacitor. In both devices, the polarization state of the ferroelectric layer is switched by applying sufficiently high voltages between the capacitor electrodes or the gate and the channel, respectively.
A further bistable device is a resistor with two (or more) reversibly switchable and persistent resistance-states. The resistor is made of a material with respective reversibly switchable and persistent conductivity states.
U.S. Pat. No. 5,448,098 describes a superconductive photoelectric device with a superconductive thin film between two electrodes, the superconductive thin film having a photo-conductive effect and converting from a normally conducting state to a superconductive state in response to light irradiation. This superconducting state can be persistent.
U.S. Pat. No. 6,204,139 describes a method for switching properties of perovskite materials used in thin film resistors. The properties, in particular the conductivity, are switched reversibly by short electrical pulses. Application of the method for non volatile memory units and sensors with changeable sensitivity is proposed.
U.S. Pat. No. 6,531,371 describes an electrically programmable resistance cross point memory. At cross points of bit lines and word lines, perovskite material acts as variable resistors, the resistance values of which can be changed reversibly and with hysteresis.
US 20030156445 A1 describes a method of changing the resistance of a perovskite metal oxide thin film device by means of a voltage pulse.
Other materials are described in the three articles and the international application publication mentioned subsequently.
The articles “Reproducible switching effect in thin oxide films for memory applications” (A. Beck et al., Applied Physics letters, Vol. 77, No. 1, July 2000), “Current-driven insulator-conductor transition and nonvolatile memory in chromium-doped SrTiO3 single crystals” (Y. Watanabe et al., Applied Physics Letters, Vol. 78, No. 23, June 2001) and “Electrical current distribution across a metal-insulator-metal structure during bistable switching” (C. Rossel et al., Journal of applied Physics, Vol. 90, No. 6, September 2001) and the international application publication WO 00/49659 A1 describe materials and classes of materials with hysteretically switchable electro static resistance, and simple resistor devices made from these materials.
The above-mentioned devices provide switching speeds, power consumptions and manufacturing costs which are still unsatisfactory for many applications.